Self-restoring current limiting device

ABSTRACT

A current terminal insulatingly disposed in a cylindrical metallic shell is normally electrically connected to the shell through a self-restoring type current limiting material filling a central fine opening extending through a porcelain insulation disposed in the shell. An increased pressure in the opening due to the evaporation of the material is absorbed by pressure release means disposed in the current terminal. The shell can be operatively connected in axially aligned relationship to another current terminal including similar pressure release means. Alternatively the shell may be connected in axially aligned relationship to its mirror image with the current limiting material normally electrically interconnecting both terminals while the shell is insulated from the terminals.

United States Patent Miyamoto et al.

SELF-RESTORING CURRENT LIMITING DEVICE [73] Assignee: Mitsubishi Denki Kahushiki Kaisha,

Japan [22] Filed: Jan. 24, 1974 [21] Appl. No.: 436,385

[30] Foreign Application Priority Data Jan. 25, 1973 Japan 48-10421 Aug. 24, 1973 Japan 48-94996 [52] US. Cl 337/119; 337/121 [51] Int. Cl. H0111 87/00 [58] Field of Search 337/114, 116, 118, 119, 337/121, 158, 159, 306, 326

[56] References Cited UNITED STATES PATENTS 3,644,860 2/1972 Yamagata et a1 337/118 X [451 May 27, 1975 Primary Examiner-G. Harris Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J. Lobato; Bruce L. Adams [57] ABSTRACT A current terminal insulatingly disposed in a cylindrical metallic shell is normally electrically connected to the shell through a self-restoring type current limiting material filling a central fine opening extending through a porcelain insulation disposed in the shell. An increased pressure in the opening due to the evaporation of the material is absorbed by pressure release means disposed in the current terminal. The shell can be operatively connected in axially aligned relationship to another current terminal including similar pressure release means. Alternatively the shell may be connected in axially aligned relationship to its mirror image with the current limiting material normally electrically interconnecting both terminals while the shell is insulated from the terminals.

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SELF-RESTORING CURRENT LIMITING DEVICE BACKGROUND OF THE INVENTION This invention relates to a structure of a current limiting device, and more particularly to a current limiting device of self-restoring type including an amount of current limiting material in the form of a solid or a liquid exhibiting a good electric conductivity at room temperature and responsive to a flow of shortcircuiting current therethrough to be evaporated to present a high impedance after which it is solidified or liquidized to restore the good electric conductivity, and a closed space having the amount of current limiting material accommodated therein.

One form of conventional current limiting devices of the type referred to has comprised a cylindrical metallic shell having a current terminal block with a collar portion electrically insulatingly disposed therein and an amount of self-restoring type current limiting material filling a central fine opening extending through a cylindrical insulation disposed in the shell, and another cylindrical current terminal block including pressure release means and having the shell screw threaded thereinto with the current limiting material electrically connecting the terminal block within the shell to both the external terminal block and the shell at room temperature.

Since such devices are manufactured by using moulding technique, the collar portion of the terminal block has been subject to a high compressive stress in excess of a maximum permissible compressive stress therefor during the cooling of the device after the moulding. Also the devices have been subject to a limitation as to the applied voltage.

SUMMARY OF THE INVENTION Accordingly it is an object of the present invention to provide a new and improved current limiting device of self-restoring type increased in a mechanical strength to a compressive stress of a current terminal block with the overall length of the device decreased.

It is another object of the present invention to provide a new and improved current limiting device of selfrestoring type operative with high voltages.

The present invention accomplishes these objects by the provision of a current limiting device of selfrestoring type comprising a cylindrical metallic shell, an electric insulation surrounded by the cylindrical shell and having a central fine opening extending throughout the same, an amount of self-restoring type current limiting material filling the opening, the current limiting material being responsive to a flow of excessive current therethrough to produce Joule heat therein to be evaporated, and a current terminal block including pressure release means for decreasing a pressure provided by the evaporated portion of the current limiting material and a collar portion radially projecting beyond the outer periphery thereof, the current terminal block engaging the cylindrical shell through the electric insulation.

Preferably the collar portion of the current terminal block may be dimensioned so that it encircles at least the overall axial dimension of a compartment forming a part of the pressure release means to be filled with an amount of the current limiting material.

A pair of current terminal blocks including the pressure release means may be disposed on both sides of the electric insulation and one of the current terminal block engages the cylindrical shell through the electric insulation. Alternatively the pair of current terminal blocks may engage the cylindrical shell through the electric insulation.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. I is a longitudinal sectional view of a current limiting device constructed in accordance with the principles of the prior art;

FIG. 2 is a longitudinal sectional view of a current limiting device constructed in accordance with the principles of the present invention;

FIG. 3 is a view similar to FIG. 2 but illustrating a modification of the arrangement shown in FIG. 2;

FIG. 4 is a view similar to FIG. 2 but illustrating a modification of the arrangement shown in FIG. 3;

FIG. 5 is a longitudinal sectional view of a modification of the present invention;

FIG. 6 is a view similar to FIG. 5 but illustrating a modification of the arrangement shown in FIG. 5;

FIG. 7 is a view similar to FIG. 5 but illustrating a modification of the arrangement shown in FIG. 6;

FIG. 8 is a longitudinal sectional view of still another modification of the present invention;

FIG. 9 is a view similar to FIG. 8 but illustrating a modification of the arrangement shown in FIG. 8; and

FIG. 10 is a view similar to FIG. 8 but illustrating a modification of the arrangement shown in FIG. 9.

Throughout the figures like reference numerals designate the identical or similar components.

DESCRIPTION OF AN EARLIER DEVICE Referring now to the drawings and FIG. 1 in particular there is illustrated a self-restoring type current limiting device constructed in accordance with the principles of the prior art. The arrangement illustrated comprises a cylindrical metallic shell 10 including a stepped cylindrical bore coaxially thereof and having a larger open end at one end thereof and a reduced diameter portion on the other end portion thereof. In the stepped cylindrical bore there are disposed a metallic buffer member 12 in the form of a shallow cup, a cylindrical insulation 14 formed, for example, of a porcelain and a current terminal block 16 including one end portion beyond the one end of the shell 10 and a collar portion 18 radially extending from another end portion of the block 16, closer to the other and reduced end portion of the bore in shell 10. An electric insulation 20 of annular cross section is disposed between the internal wall surface of the stepped cylindrical housing 10 bore and the components 14 and I6 and a metallic retaining ring 22 is screw threaded into the larger end portion of the bore to surround the adjacent portion of the cylindrical terminal block 16 through the insulation 20. A central fine opening 24 extends through the other end portion of the shell 10, the buffer member 12 and a substantial portion of the insulation 14 and communicates through a contracted opening 26 to an opening 28 substantially equal in diameter to the opening 24 and extending through the current terminal block 16. The opening 28 is connected to a larger diameter aperture opened at the end of the terminal block 16 with a needle valve 30 disposed in sealing relationship in the aperture.

Another metallic current terminal block 32 in the form of a cylinder substantially equal in diameter to the shell is provided on one end portion with a cylindrical recess substantially complementary in shape to the reduced diameter portion of the shell 10 and includes a cylindrical space coaxial therewith and opened at the bottom of the recess and communicating with a fine opening closed by a needle valve 34 disposed on the other end portion of the terminal block 32. The cylindrical space in the current terminal block 32 includes a compartment 36 having one side opened at the bot tom of the recess and the other side defined by a piston 38 movably disposed in the space. The remaining portion of the space forms a compressed gas compartment 40 filled with an amount of compressed gas 42 to apply a back pressure to the piston 38. The piston 38 is provided on the peripheral surface with an O ring 44 for isolating the compressed gas 42 from the compartment 36.

The reduced diameter portion of the cylindrical shell 10 is screw threaded into the recess on the current ter minal block 32 with a sealing gasket 46 interposed therebetween. The gasket 46 has a fine aperture communicating the compartment 36 with the fine opening 24. The openings 24, 26 and 28 and the compartment 36 are filled with an amount of self-restoring type current limiting material well known in the art. That portion of the current limiting material disposed in the openings 24, 26 and 28 is designated by the reference numeral 48 and that portion thereof filling the compartment 36 is designated by the reference numeral 48.

In operation, a current can flow from the current terminal block 10 through the current limiting material 48', 48' into the other current terminal block 32. If an excessive current such as a shortcircuiting current flows through the current limiting material in the arrangement then Joule heat is generated in the material to first evaporate that portion of the material 48 in the contracted opening 26. Then the evaporation proceeds toward the buffer member 12 until all the current limiting material 48 within the porcelain insulation 14 is evaporated. Further the current limiting material 48' in the compartment 36 is at least partly evaporated. The evaporated material presents an electric resistance approximating 1,000 times the resistance thereof at room temperature. For example, it is possible to limit a shortcircuiting current having a peak value of 400 KA to the order of KA.

More specifically, upon the occurrence of a shortcircuiting current, the current limiting material increases in volume through the evaporation. As a result, a pressure in the openings 24, 26 and 28 and in the compartment 36 increases to move the piston 38 toward the needle valve 34 against the action of the compressed gas 42 thereby to reduce the pressure of the evaporated material. Thus the compressed gas compartment 40, the piston 38 and the compartment 36 filled with the current limiting material 48' acts as pressure release means.

On the other hand, the current limited by the evaporated material is interrupted by a switching device (not shown) serially connected to the arrangement of FIG. 1. As a result, the current no longer flows and the evaporated current limiting material returns back to its original liquid or solid state while causing the piston 38 to restore its original position as illustrated in FIG. 1 by means of the action of the compressed gas 42 in the compartment 40. Therefore a current can again flow through the arrangement of FIG. 1.

The arrangement as shown in FIG. I may be produced in the manner as described, for example, in above us. patent application Ser. No. 210,772, entitled Self-Recovering Current Limiter filed on Dec. 22, 1971 by T. lnoue et al. and assigned to the same assignee as the present application. According to the cited US. application the moulding is effected at a temperature exceeding the melting point of the material of the insulation 20 after which the resulting arrangement is left to be slowly cooled to room temperature. During that cooling, the shell 10 and the terminal block 16 are subject to considerable shrinking because of their high coefficients of thermal expansion while the porcelain insulation 20 is very small in shrinkage because of its low coefficient of thermal expansion. In addition, the terminal block 16 is smaller in shrinkage than the shell 10 because the former is shorter than the latter. Therefore the collar portion 18 of the terminal block 16 is subject to a compressive stress much higher than a permissible stress therefor. In order to prevent the collar portion of the terminal block from being exposed to such an excessive compressive stress, the collar portion 18 of the terminal block may be increased in axial dimension. This causes the resulting device to increase in both the overall length and manufacturing cost.

The arrangement of FIG. 1 is also disadvantageous in that it has a limitation as to a voltage applied thereto. For example, it is required to increase the axial length of the porcelain insulation 14 with an increase in the applied voltage. The axial length of the insulation 14 is labelled by the reference character A in FIG. 1. An increase in the axial dimension of the insulation 14 causes a longer time delay with which the piston 38 will move after the current limiting material has begun to evaporate so that the ability of the piston 38 to release the pressure is reduced. This results in the necessity of greatly increasing the mechanical strength of the current limiting device. However it is nearly impossible to impart such a high mechanical strength to the device. This means that current limiting devices are, for themselves, subject to a limitation as to the applied voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention contemplates to increase the mechanical strength to the compressive stress of the current terminal block 16 and to decrease the overall length of the current limiting device. Further it contemplates to provide a current limiting device operative with high voltages.

Referring now to FIG. 2, there is illustrated a current limiting device constructed in accordance with the principles of the present invention. The arrangement illustrated comprises a cylindrical metallic shell 210 generally similar to that shown in FIG. 1. Namely, the shell 210 includes a stepped cylindrical bore having disposed therein a metallic buffer member 12 in the form of a shallow cup, a cylindrical insulation 14 formed, for example of a porcelain material and a current tenninal block 216 projecting beyond one end thereof. The components 12, 14 and 216 are disposed in coaxial relationship with the shell 210, generally as in the arrangement of FIG. 1. An electric insulation 20 of annular cross section is disposed between the internal wall surface of the bore in stepped shell 210 and the components 12, 14 and 216. A retaining ring 22 is screw threaded into the open end portion of the bore to surround the adjacent portion of the current terminal block 216 through the insulation 20. A central fine opening 24 extends through the buffer member 12 and the insulation 14 into the current terminal block 216 and is filled with a current limiting material 48.

The current terminal block 216 has therein a cylindrical space or bore similar to that disposed in the terminal block 32 shown in FIG. 1, and the fine opening 24 communicates with one end of this cylindrical space. The other end of this cylindrical space projects from the shell 210 and is closed by a closure 50 screw threaded into the projecting end portion of the block 216. The bore includes a compartment 36 filled with the same current limiting material 48' as that in the opening 24. The bore also provides space for a piston 38 and a compartment 40 filled with a compressed gas 42. Compartment 40 is closed by a needle valve 30 disposed in the closure 50. Thus the device of FIG. 2 has pressure release means generally similar to that shown in FIG. 1 but disposed in the current terminal block 216. The piston 38 is provided on the outer periphery with an O-ring 44 for isolating the compressed gas 42 in the compartment 40 from the current limiting material 48' in the compartment 36.

The current terminal block 210 has a collar portion 218 radially extending from that end portion thereof adjacent the insulation 14.

The cylindrical shell 210 can be formed of a metallic material selected from the group consisting of stainless steels, high tension brasses and hard copper alloys.

The buffer member 12 is preferably formed of a soft metal such as copper in order to decrease an impact applied to the insulation 14 due to a compressive stress caused from a contraction of the cylindrical shell which is likely to occur upon the moulding of the device.

The current terminal block 16 can be made of copper or alloy thereof such as chromium-containing copper alloy.

As above described, the cylindrical insulation 14 can be formed of a porcelain material. The insulation 14 may be produced by moulding an electrically insulating material required only to withstand a pressure applied thereto upon the moulding as well as a high pressure and a high temperature imposed thereupon during the current limitation. The material is required to have high resistance against corrosion by the associated current limiting material. Alumina, beryllia or the like adequately meets such requirements.

The insulation 20 is preferably of an electrically insulating inorganic material. The most suitable material for the insulation 20 is a so-called moulded mica produced by mixing a powder of natural or synthetic mica with a powder of low melting-point glass, charging the resulting mixture in a mould, heating the mixture with the mould to about a temperature at which the glass is fused and softened, and applying a pressure to the mixture maintained in the heated condition.

The current limiting material 48, 48 is of the selfrestoring type and in the form of a liquid or a solid exhibiting a good electric conductivity at room temperature and responsive to a flow of excessive current such as short-circuiting current therethrough to be evaporated by means of Joule heat generated therein to present a high impedance. After the interruption of the current, the evaporated material is left to be cooled to room temperature to become liquid or solid restoring the good electric conductivity. The current limiting material in the form of a liquid or a solid electrically connects the current terminal block 216 to the cylindrical shell 210. Preferred examples of the current limiting material include sodium (Na), potassium (K), and a solid solution of sodium and potassium. Further information may be found. for example, in U.S. Pat. No. 3,501,730 entitled Current Limiting Equipment" issued on Mar. 17, 1970 to T. Ito et al. and assigned to the same assignee as the present application.

The compressed gas 42 is preferably inert.

The arrangement as shown in FIG. 2 can be produced by first inserting the buffer member 12, the porcelain insulation 14 and the current terminal block 16 into the cylindrical shell 210 in the named order to form an annular space, and then filling the annular space with an electrically insulating material 20 for example an inor ganic insulating material under pressure, at a temperature exceeding its melting point, after which the assembly is left to be slowly cooled to room temperature.

During the cooling, an axial compressive stress due to contraction of the cylindrical shell 210 is applied to the collar portion 218 of the terminal block 216. Also, a radial compressive stress due to the contraction of the shell 10 is applied to the cylindrical insulation 14 through the insulation 20 and acts remains, as a residual compressive stress, in the insulation 14 at room temperature. The residual compressive stress acts to increase the mechanical strength of the cylindrical insulation 14.

The arrangement of FIG. 2 is operated in the same general manner as above described in conjunction with FIG. 1.

As above described, the arrangement of FIG. 2 has the pressure release means consisting of the compartment 36 filled with the current limiting material 48', the piston 38 and the compressed gas compartment 40 are disposed in the current terminal block 16. Therefore the current terminal block 32 shown in FIG. 1 is omitted and instead the closure 50 is disposed in the end portion of the terminal block 16 to close one end of the compressed gas compartment 40. Therefore it will be appreciated that the arrangement of FIG. 2 is much reduced in the axial dimension as compared with that shown in FIG. 1.

In FIG. 2 it is seen that the current terminal 16 includes the collar portion 18 having an axial dimension insufficient to enclose the entire length of the compartment 36 filled with the current limiting material 48'.

FIG. 3 shows a modification of the arrangement illustrated in FIG. 2 wherein the current terminal block 316 includes collar portion 318 having an axial dimension sufficient to enclose the total of the axial dimensions of the compartment 36 and the piston 38.

In order that the collar portion 318 of the terminal block 316 can sufficiently withstand a compressive stress due to a contraction of the housing cylindrical shell 10 during the moulding of the device, the collar portion 318 has preferably an axial dimension sufficient to encircle at least the entire length of the compartment 36 filled with the current limiting material 48 and specifically the axial dimension thereof plus an additional axial dimension of the compartment 36 expanded through the movement of the piston 38 due to the evaporation of the current limiting material in the opening 24 and at least one portion of the current limiting material in the compartment 36 upon a flow of short-circuiting current therethrough. It has been found that collar portion 318 having such an axial dimension can sufficiently withstand a compressive force due to a contraction of the shell 310 occurring during the moulding.

Also the shell 310 as shown in FIG. 3 is larger in axial length than that illustrated in FIG. 2 so that its end portion completely encircles the compressed gas compartment 40. However, an arrangement similar to that of FIG. 3 can be equal in overall length to that shown in FIG. 2. In any event it can be decreased to about twothirds the entire length of the conventional arrangement as shown in FIG. 1.

In the arrangements shown in FIGS. 2 and 3, upon the evaporation of the current limiting material, the piston 40 may sometimes strike against the sealing closure 50 thereby to reduce a sealing surface pressure provided by the sealing closure 50. This results in escape of the compressed gas 42 from the compartment 40 through or around the closure 50. In order to eliminate this escape of compressed gas, a stopping limiter or piston stop member 52 in the form of an annulus can be fixedly disposed in the compressed gas compartment 42 as shown in FIG. 4. The stop member 52 limits the righthand movement as viewed in FIG. 4 of the piston so as to prevent the piston's striking against the closure 50. In other respects, the arrangement is substantially identical to that shown in FIG. 3.

Referring now to FIG. 5, there is illustrated a modification of the present invention. In FIG. 5 it is seen that a current terminal block 532 such as shown in FIG. 1 by the reference numeral 32 is operatively coupled in an axially aligned relationship to an arrangement similar to that illustrated in FIG. 2. More specifically the latter arrangement is different from the arrangement as shown in FIG. 2 mainly in that a cylindrical insulation 514 has an axial dimension approximately equal to twice that of the insulation 14 shown in FIG. 2 and includes a fine opening 524 extending therethrough and having a contracted opening 526 in the middle portion thereof with the needle valve 32 as shown in FIG. 2 omitted. As compared with the insulation 14 as shown in FIG. 1 the insulation 14 shown in FIG. 5 is also dou bled in the axial dimension.

The cylindrical shell S is screw threaded into the current terminal block 532 through an apertured sealing gasket 46 in the same manner as previously described in conjunction with FIG. 1.

The resulting arrangement is suitable for use in high voltage applications and has an axial dimension substantially equal to twice that of the arrangement as shown in FIG. 1 plus the contracted opening 526 formed in the middle portion of the fine opening 524, connected to a pair of pressure release means disposed on both sides of the cylindrical insulation 514. It is noted that the shell 510 engages only the current terminal block 516 through the cylindrical insulation 514.

As compared with FIG. 1, it is seen in FIG. 5 that the lefthancl portion of the arrangement as viewed from the central opening 526 is substantially equal in construction to the corresponding portion shown in FIG. 1 while the righthand portion thereof is substantially similar to the arrangement of FIG. 2.

Upon the occurrence of an excessive current such as a short-circuiting current, that portion of the current limiting material in the central contracted opening 526 is first evaporated followed by the evaporation proceeding toward both ends of the opening 22. Thereby the lefthand piston 538 as viewed in FIG. 5 is moved toward the needle valve 34 while the righthand piston 538' is moved toward the needle valve 30. Thus the pressure release is effected on both sides of the contracted opening 526. As a result, the severity of mechanical impact remains unchanged between the arrangements shown in FIGS. 1 and 5, although the arrangement of FIG. 5 has the cylindrical porcelain insulation l4 doubled in length as compared with that shown in FIG. 1.

FIG. 6 shows an arrangement 610 substantially identical to that illustrated in FIG. 5 excepting that the collar portion 618 of the current terminal block 16 is increased in the axial dimension as in the arrangement of FIG. 3.

An arrangement 710 as shown in FIG. 7 is identical to that illustrated in FIG. 6 excepting that a stopper 752 is disposed in the compressed gas compartment 40 as in the arrangement of FIG. 4.

Referring now to FIG. 8, there is illustrated still another modification 810 of the present invention. The arrangement illustrated includes the righthand half of the arrangement as shown in FIG. 5 and a mirror image thereof connected together into a unitary structure. Therefore it has a pair of pressure release means, including valves 30' of the same construction as above described disposed in bisymetrical relationship on both sides of the porcelain insulation 514. The insulation 514 has an axial dimension substantially equal to twice the axial dimension of the insulation 14 as shown in FIG. 1. It is noted that the cylindrical shell 810 engages both current terminal blocks 516, 516' through the porcelain insulation 20 and is electrically insulated from the terminal blocks 516, 516'.

The arrangement has, in addition to the above described advantages of that shown in FIG. 5, the advantage that the cylindrical shell 810 can be used to mount and/or fix the arrangement to another member without a separate insulation such as an insulator or insulators interposed therebetween. This is because the shell 810 is fully electrically isolated from a current path extending from one to the other of the current terminal blocks 516, 516'.

FIG. 9 shows an arrangement substantially identical to that illustrated in FIG. 8 excepting that the each current terminal block 16 includes a collar portion 818 the axial dimension of which is greater than that of the terminal blocks 516, 516' shown in FIG. 8 as in the arrangement of FIG. 3.

An arrangement as shown in FIG. 10 is different from that illustrated in FIG. 9 only in that one annular stopper 952, 952' such as shown in FIG. 4 is disposed in each of the gas compartments 940, 940'.

The arrangements as shown in FIGS. 5 through 10 exhibit the effect of decreasing the axial dimension over the conventional arrangement as illustrated in FIG. 1. While those arrangements are equal in the outside diameter to the conventional arrangement, the cylindrical insulation 514 has an axial dimension equal to twice the axial dimension of the cylindrical insulation I4 disposed in the conventional arrangement of FIG. 1. Therefore the arrangements of the present invention as shown in FIGS. 5 through may be compared with the conventional arrangement doubled in the axial dimension, that is, the arrangement of FIG. 1 having axially connected thereto an arrangement represented by phantom line shown on the lefthand portion of FIG. 1.

In FIGS. 1, and 5 it is assumed that A designates the axial length of the cylindrical insulation 16, B designates an axial length between the ends of the current terminal block 32 and the insulation 16, C designates the axial length of the current terminal block 16 and D designates the axial length of the modified righthand end structure of FIG. 5. Then the arrangement of FIG. 1 doubled in the axial length has an axial length expressed by 2A 2B 2C, as shown. With the symbols A, B, C and D applied to the corresponding portions shown in FIG. 8, the arrangement of FIG. 8 has an axial length expressed by 2A 2D. Since B, C and D are substantially equal to one another, the arrangements as shown in FIGS. 5 through 10 have an axial length shorter than that of the arrangement of FIG. 1 doubled in the axial length by about 2C. This decrease in the axial length corresponds to about 30 percent of the entire axial length of the arrangement of FIG. 1.

The present invention has several advantages. For example, the current terminal block 16 can sufficiently mechanically withstand a compressive stress developed therein due to a contraction of the housing 10 during the moulding of the device. The overall axial dimension can be shorter about 30 percent than that of conventional devices as long as the applied voltage remains unchanged therebetween. A maximum permissible voltage applied to the device can be doubled by doubling the length of the cylindrical insulation 14 with the overall length of the device maintained small. For example, the overall length can be smaller about 30 percent than the double length of conventional devices. With the pressure release means disposed on both end portions of the device, the severity as to the mechanical strength can be decreased even with the applied voltage maintained low.

While the present invention has been illustrated and described in conjunction with various preferred embodiments thereof it is to be understood that numerous changes and modifications may be resorted to without departing from the spirit and scope of the present invention. For example, the pressure release means is not limited to that illustrated and may be of any suitable type such as a bellows type disclosed in the above cited US. Pat. No. 3,50l,730.

What we claim is:

l. A self-restoring current limiting device, comprismg;

a metallic shell having first and second ends; an electric insulator surrounded by and in contact with the inside of said shell and having an opening extending from said first to said second end; a body of current limiting material filling said opening, said material being in a selected one of original, solid and liquid states at room temperature, being responsive to a llow of electric current therethrough in excess of a predetermined magnitude by evaporation of the material, and being self-restoring to its original state when the excess current terminates;

a current terminal block including pressure release means for releasing a pressure of evaporated current limiting material, said pressure release means including a compartment communicating with said opening and filled with an additional amount of current limiting material; a movable piston defining one side of said compartment; and

means for applying a back pressure to said piston.

2. A current limting device as claimed in claim 1 wherein said current terminal block has a collar portion radially projecting from said block, engaging said shell through said insulator and so dimensioned that it substantially encircles said compartment.

3. A current limiting device as claimed in claim I, wherein said means for applying back pressure com prises a compressed gas compartment, and stop means in said compressed gas compartment for limiting movement of said piston in response to said pressure.

4. A self-restoring current limiting device, comprising;

a metallic shell having first and second ends; an electric insulator surrounded by and in contact with the inside of said shell and having an opening extending from said first to said second end, at least a central portion of said opening between said ends being a fine aperture; an amount of current limiting material filling said opening, said material being in a selected one of original, solid and liquid states at room temperature, being responsive to an excessive flow of electric current therethrough, in excess of a predetermined magnitude, by evaporation of the material, and being self-restoring to its original state when the excessive flow of current terminates;

a pair of current terminal blocks including pressure release means for releasing a pressure of evaporated current limiting material, each pressure release means comprising a terminal casing of larger diameter than said opening, communicating said opening, and filled with a further amount of current limiting material; a movable piston in said terminal casing defining one side of said terminal casing filled with current limiting material; and means for applying a back pressure to said piston.

5. A device according to claim 4 wherein said metallic shell engages at least one of said current terminal blocks through said insulator to hold the respective terminal casing to said shell.

6. A device according to claim 4 wherein said shell engages both current terminal blocks of said pair through said insulator to hold the respective casings to said shell. 

1. A self-restoring current limiting device, comprising; a metallic shell having first and second ends; an electric insulator surrounded by and in contact with the inside of said shell and having an opening extending from said first to said second end; a body of current limiting material filling said opening, said material being in a selected one of original, solid and liquid states at room temperature, being responsive to a flow of electric current therethrough in excess of a predetermined magnitude by evaporation of the material, and being self-restoring to its original state when the excess current terminates; a current terminal block including pressure release means for releasing a pressure of evaporated current limiting material, said pressure release means including a compartment communicating with said opening and filled with an additional amount of current limiting material; a movable piston defining one side of said compartment; and means for applying a back pressure to said piston.
 2. A current limting device as claimed in claim 1 wherein said current terminal block has a collar portion radially projecting from said block, engaging said shell through said insulator and so dimensioned that it substantially encircles said compartment.
 3. A current limiting device as claimed in claim 1, wherein said means for applying back pressure comprises a compressed gas compartment, and stop means in said compressed gas compartment for limiting movement of said piston in response to said pressure.
 4. A self-restoring current limiting device, comprising; a metallic shell having first and second ends; an electric insulator surrounded by and in contact with the inside of said shell and having an opening extending from said first to said second end, at least a central portion of said opening between said ends being a fine aperture; an amount of current limiting material filling said opening, said material being in a selected one of original, solid and liquid states at room temperature, being responsive to an excessive flow of electric current therethrough, in excess of a predetermined magnitude, by evaporation of the material, and being self-restoring to its original state when the excessive flow of current terminates; a pair of current terminal blocks including pressure release means for releasing a pressure of evaporated current limiting material, each pressure release means comprising a terminal casing of larger diameter than said opening, communicating said opening, and filled with a further amount of current limiting material; a movable piston in said terminal casing defining one side of said terminal casing filled with current limiting material; and means for applying a back pressure to said piston.
 5. A device according to claim 4 wherein said metallic shell engages at least one of said current terminal blocks through said insulator to hold the respective terminal casing to said shell.
 6. A device according to claim 4 wherein said shell engages both current terminal blocks of said pair through said insulator to hold the respective casings to said shell. 